It has been common in the prior art to construct underground supply and transport conduits or ducts which are not passable to a person i.e. not manually accessible, by open construction methods, that is, a ditch is dug, the pipe, duct or conduit is assembled in the ditch, and subsequently the ditch is filled.
Because the need for subterranean conduits is continuously increasing in built up areas these open construction methods, particularly in city environments, completely or partly block many city streets because of the excavations necessary for the conduits, pipes or ducts, for weeks on end. Because of this inconvenience it is desirable to resort to an underground or closed construction method.
In this kind of construction a shaft is formed and the subterranean passage is produced laterally from this shaft toward a target shaft by an excavating head with a conveyor screw or with means for flushing away the detritus for example to a maximum length of 80 meters. The product pipe often made of steel, is forced into this passage to form the liner.
The pressing operation whereby the liner is forced into the passage is a problem because of the long lengths and high resistance so that especially heavy machinery is required for this purpose. The wall thickness of the liner must also be comparatively great so as to accommodate such forces.
The essential disadvantages of this kind of process for laying product conduit or ducts in underground or closed construction methods, therefore, lie in the excavation length and the force gradient factors which limit the applications in which the technique may be used.
The development of processes and apparatus for excavating extended holes for underground ducts, conduits and the like has reached the point that today one can make use of controllable excavation processes, which are remotely-controlled. Dirigible cutting heads can be guided through subterranean structures with commercially available control equipment.
The major difficulty here is the lining of the extended hole with a fluid-impermeable structure or to provide this type of extended hole with a similar structure which can act as a duct, conduit or the like.